use cl.ID instead of pk.Connect.ClientIdentifier when looking for existing clients in inheritClientSession (#417)

* use cl.id instead of pk.Connect.ClientIdentifier which is client-supplied, when looking for existing clients in inheritClientSession

* look for assigned client id explicitly

* simpler implementation using just cl.ID

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Co-authored-by: JB <[email protected]>

Author: SimonMacIntyre

server.go

@@ -560,7 +560,7 @@ func (s *Server) validateConnect(cl *Client, pk packets.Packet) packets.Code {
 // connection ID. If clean is true, the state of any previously existing client
 // session is abandoned.
 func (s *Server) inheritClientSession(pk packets.Packet, cl *Client) bool {
-	if existing, ok := s.Clients.Get(pk.Connect.ClientIdentifier); ok {
+	if existing, ok := s.Clients.Get(cl.ID); ok {
 		_ = s.DisconnectClient(existing, packets.ErrSessionTakenOver)                                   // [MQTT-3.1.4-3]
 		if pk.Connect.Clean || (existing.Properties.Clean && existing.Properties.ProtocolVersion < 5) { // [MQTT-3.1.2-4] [MQTT-3.1.4-4]
 			s.UnsubscribeClient(existing)